The present invention relates to a shaft for supporting cut roll portions in a cutting-reeling machine and to a cutting-reeling machine comprising such a shaft.
The term cutting-reeling machine as used herein means a machine which can unwind a roll of strip material, for example, paper or plastics film, cut it perpendicular to its axis into two or more portions and rewind the cut portions forming rolls of smaller axial length than the starting roll.
The cut portions are usually rewound on a pair of separate shafts so that adjacent portions of the original roll are rewound on different shafts, staggered in a chessboard-type arrangement. In particular, the various portions are rewound on respective tubular cores, usually of cardboard, positioned coaxially around the shafts.
More particularly, the present invention relates to a shaft for supporting cut roll portions, comprising a cylindrical body the outer surface of which has at least one recess for housing a resilient envelope which can be connected to a source of fluid under pressure, for example compressed air, by ducts in the cylindrical body.
According to the prior art, the outer surface of the cylindrical body of such a shaft has a plurality of uniformly-spaced longitudinal circumferential recesses each of which can house a respective resilient envelope which can be inflated as a result of the admission of compressed air to its interior.
The inflation of the envelopes causes their outer surfaces to expand and to be pressed against the inner surfaces of the cardboard cores onto which the cut roll portions are rewound, preventing them from sliding relative to the shaft during the rotary motion thereof.
If the thickness of the material of the original roll is not constant throughout the width of the strip, for example, owing to manufacturing defects, the rewinding of the cut portions causes their radial dimensions to vary relative to one another, naturally being greater for the portions cut from the thicker portions of the original roll.
At the same time, the angular velocity of the rotation of all of the roll portions which are rewound on the same shaft is equal since the direct contact between the inflated envelopes associated with the shaft and the cores of the various cut roll portions prevents any relative sliding.
As a result of this, at a given moment, the tangential winding velocities of the strip material in the various cut roll portions supported by the same shaft may be different. In fact, these tangential velocities result from the product of a constant angular velocity and a radius which may vary as a result of the possible variations in thickness mentioned above.
Different tangential velocities are extremely harmful since they result in different tensions in the various cut portions of strip being rewound. At the least, some of these tensions will in fact differ from the theoretical value causing the rewinding of the cut roll portion concerned to be loose if they are too low, or too tight if they are too high.